Back to the AR

[Clark Howard]

After my experiences with the M-16 in RVN, I have not had much use for the rifle for some years. Lately, I have been thinking that a light, short AR with a 20 round mag might be handy in the truck. Some of you guys can surely fill me in on the practical effects of rifling twist on accuracy. What twist rate will best stabilize bullets from 55gr. to 67gr. I tried some M855 in an older AR, and it threw bullets all over the target, while it shot the 55's very well. What twist will give me decent accuracy with both? Thanks for your thoughts. Regards, Clark

[joem]

The older AR most likely had a 1/12 twist and mine shoots the 55 gr very well. If you don't reload you might want a 1/9 twist that should shoot 55 and 62 and maybe 65 pretty well. My 1/9 does better with 62. A 1/7 needs 69 gr Nosler while a 1/8 needs 77 gr match bullets. Don't ask me why, I can't figure it out either. Let us know what you get and how it shoots.

[gwp]

This is a very good explanation of stabilization of the .223 bullet.

http://anarchangel.blogspot.com/2007...mythology.html

This is a long read but this extract from the article should help.

I haven’t seen a 55gr milsurp disintegrate from overspin in a 1:7 barrel, but there is a small but clear reduction in accuracy at long range between a 1:9 barrel and a 1:7 barrel with the 55gr bullets.

62gr bullets will partially stabilize with 1:12 to the point where they will group at 25-50 yards, and generally at least hit the target at 100 yards (though the groups will be more like shotgun patterns), but really need 1:10 to fully stabilize. They will not overspin with any barrel I’ve seen.

Most 68-69gr bullets will do all right with 1:9 but do better at long range with 1:8; though some will stabilize at 1:10.

Most 70-72gr will also stabilize at 1:9, but do better with 1:8.

Some 72-75gr bullets will do well with 1:9, some need 1:8.

Bullets heavier than 75gr really need 1:8 minimum and prefer1:7.

77-82gr bullets generally need a minimum of 1:7 (though the 77gr SMK will at least partially stabilize at 1:9 and do reasonably well with 1:8) ; and I’ve even seen 1:6 barrels. I know there are 1:4 barrels, but I've never seen one.

[PhillipM]

M855 does not group well in my 1:7.7 twist Krieger and I doubt it groups with anything.

https://www.full30.com/video/8e760eb...419c413b932444

[bd1]

M855 is utility-grade ammo, best barrel, best conditions. We expect it to be AVAILABLE. I'd as soon shoot pellets for accuracy, but I keep a large stash of M855 as insurance against Washington, D.C.

[Chris F]

I think people over think the over spinning part of the equation. The benchresters will tell you that you want a slow a twist as absolutely necessary to delivery the bullet point forward reliably. They're worried about minute imbalances in the bullet core/jacket being exacerbated by overspinning. If you're looking for that last 0.05" off the bench, then follow their lead. In my type of shooting, I don't see the difference. For Service Rifle competition, we're shooting 8, 7 or in some cases 6.5 twists and some numbers in between. Secret: They all shoot 55's just fine. Accuracy realized is more related to the variety of 55's (fmj vs sp vs HP) than the twist. Green Tip (M855 62's) don't shoot great no matter what twist you're using. But they're capable of hitting at 500 yards. I've seen Marines clean the Baker Target at 500 yards with M855 and their stock A2's with 7 twist barrels. In fact I ran a practice match where some High Master marines ran their A2's and M855 against civilians and their 69's and 80's and cleaned their clocks. It's all about knowing your group center and centering that up.

Years ago I bought a cheap 9 twist A2 upper as a project to show folks that you don't need the latest/greatest to go shoot Highpower. It delivered 75's and 80's point foward with accuracy at 600 yards (okay 80's were lucky...many 9's won't shoot them). On the other end of the spectrum, I've got a couple of 6.5 twist barrels I got for shooting 90's. I got 3300fps+ with 52's and none of them went "poof".

For a truck rifle, get a 9 or better (whatever you find first and cheapest) and go shoot.

[Bill Wylde]

Hello, Chris. The Canadian C-77 62 grain ammo was superb in anything. Not sure about the current C-77, but in the old dayz that M-855 type ammo was excellent.

I've played with a lot of bullets and a lot of twists. My statement remains that it is very hard to over spin a good bullet.

[Chris F]

Hey Grandpa Yoda! Good to see you still posting and sharing the knowledge. Thanks to you, I spent about 20 years chasing C-77 without ever getting my hands on any. I did get some ss109. Some RG shot good and most did not. I guess I should just sit tight and watch for the Mk262, Mk318 and M855A1 to become available if I want to grab and go.

I was just thinking about you while typing my above post regarding barrel twists. I have one of your 8.2 twist obermeyers that shoot 77's and 80's just fine to 600 yards contrary to what some of the so-called experts have been saying.

[Bill Wylde]

You just never visited my basement, Chris. When I would go to Ottawa, several cases would be waiting. When my friends came South, they knew the entry fee. My Grandchillens will have to work hard to get rid of it!

The SR 8.2 would have been for the 69s and some of Jimmy Knox's 70-75 VLDs. You have probably never hit cool weather with that 8.2.......I was really fortunate in those development days to have Boots Obermeyer and Jack Krieger. The bullets were coming fast and those gentlemen were coming fast with the barrels. I remember well one time when I asked Jack Krieger to make some .218 bore barrels. He was screaming about having to buy a new reamer. If you were to ask him today, wonder what he might say? Boots had already made some .217 bore barrels, so was good to go with that. Those barrels just plain worked! The end for me was 7.7" twist for the SR and 8" for the MR.

Most of the SS-109 type ammo was complete garbage. I still have a bit of all of it, thinking it might mean something to history at some point. Problem with that is that few care.

[Chris F]

You've sent me the photo's of your workbench. I'm not sure I'm up for the basement. I will however jump at an invitation to your range! Give a call when you are ready to send some C77 further south to me! I promise not to link it! I think you know I never get cool weather...

Speaking of ball ammo surprises. My CIII friend was linking cheap Pakistani 7.62 ball until he happened to shoot it out of his 40X. He had to de-link a bunch and saves it for long-range! I'll see if I can send you some video.